In recent years, with the advancement of electronic technology, reduction in size and in weight, and high reliance of instruments (e.g. those for communication and defense industries), appliances and the like, and simplification of packaging systems have been required. A printed circuit board having light and flexible plastic film as insulating base board has long been desired.
The plastic films suitable for use in FPC include polyester films and polyimide films. The polyimide films exhibit excellent solderability and other physical properties and have become the main stream of FPC substrates.
It is well-known in the art that many adhesives do not bond well to polyimide films. Conventional adhesives having commercially acceptable adhesion to bond the polyimide films to metal foils (such as copper, aluminum, and the like) are acrylonitrile-butadiene copolymer, butyral resin, epoxy resin, nylon/epoxy resin, acrylonitrile-butadiene/phenolic resin, acrylics/epoxy resin, etc. However, the use of flammable organic solvents such as toluene, methylethyl ketone, acetone, etc. is required in these adhesive compositions. This could cause several disadvantages. For example, evaporation of organic solvents during the coating process needs special precautions to avoid fire hazard and air pollution; or the solvent recovery system adds extra cost.
Water-based adhesives will solve the aforementioned problems. However, to find an aqueous-type adhesive having balanced properties of adhesiveness, thermal stability, flexibility, solderability, dimensional stability, solvent resistance, flow property and electrical property remains to be a challenge. A latex of an acrylic terpolymer disclosed in U.S. Pat. No. 3,900,662 represents an example. However, a crosslinking agent must be added in the latex. Moreover, the polyimide film on which the latex is to be coated requires surface pre-treatment such as one with an alkaline solution as taught in U.S. Pat. No. 3,728,150. This adds extra cost and increases the possibility of contamination.
Japanese Patent Laid-Open No. 60,118,781 discloses that a water soluble epoxy compound and an emulsified epoxy resin are mixed with an acrylic latex to form an adhesive composition useful in FPC. However, the mixing procedure is time-consuming and will result in difficulties in quality control.